Thursday, March 24, 2005

IR-Visible Comparison

T asked in a comment about doing IR in Photoshop, and I mentioned that materials tend to reflect infra-red in an unpredictable manner. Here's a little experiment I just did, mostly to satisfy my own curiosity, but also by way of explaining this a bit further. I took the IR photos on my Cybershot, and the normal photos on my D70

The top left photo shows my Geocaching patch in all its green glory, on my green bag. In IR, both the dark green of the circular logo, and the khaki green of the cross, shine white, reflecting lots of IR. They both look equally white, so there's no difference as far as IR is concerned. The slightly less dark shade of khaki on the bag, however, is greyish, so it actually reflects less IR (despite being a 'lighter shade' in visible light). The black border of the logo and the main background also turn out black in IR - but the black tag of the bag (on the left, blurry due to my bad focusing) shines grey-white - so here we have two things that are equally black in visible light, but which reflect IR very differently.

On the bottom left is a shot of a pink and blue tissue box, a red biscuit tin, and red shoes. In IR, the colours all disappear on the tissue box: they all look equally white. As white, in fact, as the red tin. The red fabric of the shoe is white, reflecting IR, but the cross straps, which look a similar dark red, don't. The tip of the shoe, a dark red, also does not shine white under IR. Skin tones, looking at the lovely set of ankles in the shot, also tend to come out a ghostly white.

The point is that it's probably very difficult to replicate an IR shot in Photoshop, because there's no way to predict how a material reflects IR. Certainly not by colour anyway. Perhaps there's a guide by material - something along the lines of "nylon is reflective, but cotton isn't" etc. If one is shooting trees alone, however, then my earlier comment about turning all greens to white probably does the trick.